The present invention relates to an LSP sound synthesizer which can singly synthesize a human voice and the notes of birds or insects, which have different distribution functions, resulting in a high quality effect.
In the field of sound synthesizing, a line spectrum pair (LSP) system has been developed and put to practical use. The LSP can synthesize sounds with a less amount of the sound data than the known PARCOR system, and can keep the synthesized sound at a predetermined quality level or at a higher level.
In synthesizing voices by the LSP system, the voices are nonlinearly quantized using the distribution functions of LSP parameters which differ with the kinds of the voices used. The quantized voice data is used for synthesizing the original voices. This method, which quantizes the sounds using the distribution functions of LSP parameters, is advantageous is that a small number of bits are required for forming a specific sound, that is, the sound information is reduced. However, it has some disadvantages.
When sounds with different distribution functions of LSP parameters, for example, a human voice and the notes of birds or insects, a quality of the sound synthesized by a single synthesizer is remarkably poor. FIG. 1A shows a graph illustrating voice distribution functions obtained from LSP parameters w of a female voice. FIGS. 1B and 1C show sound distribution functions of the notes of a bird and an insect, respectively. As seen from these figures, characteristic curves representing voice distribution functions of the human voice are uniformly distributed with respect to frequency. If the case of the bird's notes, the characteristic curves are distributed mainly in the middle frequency region. In the case of the insect's notes, the characteristic curves are chiefly in the high frequency region. Obviously, the frequency distributions of the characteristic curves differ according to the kinds of sounds used. Hardware for the synthesizer, which is designed so as to nonlinearly quantize voices using voice distribution functions based on the frequency distributions of a human voice, requires some modification if it is used to achieve hardware for the nonlinear quantization of the notes of birds or insects. If it is applied for such without any modification, the conversion of the LSP parameters w is inevitably accompanied by the inversion of the parameters. This parameter inversion deteriorates the synthesized sound quality so much that the synthesized and reproduced notes of birds or insects have no resemblance to the original ones. For these reasons, the technique for synthesizing the human voice and the notes of birds or insects by means of a single synthesizer is considered inappropriate.